The pressure drop of cigarette filter rods, or resistance to air flow through the filter rod, is one of the primary physical properties which must be controlled in order to produce quality cigarettes. Since the pressure drop of filter rods is directly related to filtration efficiency, the tobacco industry needs assurance that accurate measurements can be made on the particular filter rods being manufactured.
"Pressure drop" involves measuring the pressure difference between the two ends of the tested filter rod, when air or other gas is drawn through it at a given flow rate. The flow rate is conventionally 17.5 milliliters per second measured at exit end of filter rod, and the air or gas may be aspirated or blown through the filter rod.
Generally, pressure drop of conventional filter rods wrapped with non-porous plugwrap has been tested on a variety of pressure drop sensing devices, using both forced air and vacuum systems. The increased use of porous plugwrap, however, has necessitated the need for totally encapsulating these filter rods with an elastic sleeve or a rubber diaphragm arrangement to prevent air leakage problems. It has been found that the pressure drops of filter rods made with the same denier per filament and total denier tow and at the same circumference and length but wrapped with paper of different porosities will vary according to the porosity of the paper. The filter rods wrapped with the less porous paper will have a higher pressure drop than the filter rods wrapped with the more porous paper. Therefore, total encapsulation of the filter rod for testing pressure drop eliminates the factor of the porosity of the paper.
A soft wrapped filter rod is one which may have a lower total denier of tow, or smaller denier per filament or a lower amount of plasticizer content, any or all of which determines the hardness or softness of a paper wrapped filter rod. A paper wrapped filter rod of sufficient hardness is normally not significantly compressed in a total encapsulation procedure.
Nonwrapped cigarette filter rods, such as nonwrapped cellulose acetate filter rods (NWA) present a different problem in accurately measuring pressure drop because without the paperwrap the elastic sleeve or rubber diaphragm actually compresses the wrapless filter and thus causes an overstatement of the pressure drop. One well-known method for testing pressure drop of nonwrapped filter rods involves inserting 50 millimeters of a filter rod into a steel tube of known inside diameter and then measuring the pressure drop by inserting the tube into a conventional pressure drop measuring apparatus. This latter method, however, has been found to be vulnerable to air leakage between the peripheral surface of the filter rod and the inside peripheral surface of the tube.
When a nonwrapped or soft wrapped filter rod is totally encapsulated the resulting compression increases pressure drop reading. A tobacco manufacturer cannot determine, therefore, the yield of the filter rod, the "yield" being a relative measure of the amount of tow required to produce a given or desired pressure drop.